Drag reduction during the side-by-side motion of a pair of intruders in a granular medium

When multiple intruders move through a granular material, complex grain-mediated interactions can arise between them. In this study, we experimentally investigate the case of two spheres moving side by side in a granular medium, focusing on how their interaction affects the drag force experienced by each sphere. Our results show that the presence of a neighboring sphere reduces the drag on the first sphere, with this reduction becoming more significant as the spheres are closer to each other. Furthermore, the amount of relative drag reduction is observed to increase with the intruders' depth in the granular medium.